New, high-performance high-efficiency compact heat-exchangers are being developed for new distributed power or combined heat and power technologies, such as microturbines, polymer-exchange membrane fuel cells, Stirling engines, gas-cooled nuclear reactors, etc. These power technologies often require thin-section austenitic stainless steels. Currently, stainless steels of types 347, 321, 304, 316 are used, but are limited by their lack of both creep-rupture resistance and corrosion resistance at 700° C. and above, especially with alternate and/or opportunity fuels and more corrosive exhaust environments. Such stainless steels also lack aging resistance and can loose ductility at low temperatures after aging. Ductility is very important for crack resistance during rapid cycling or thermal shock applications.
For extremely aggressive corrosion environments (for example, alternate fuels containing sulfur and fuel-reforming to produce hydrogen for fuel cells that add carburization and/or dusting to corrosion attack mechanisms) at 800° C. or above, alloys capable of forming protective alumina scales would be even better than alloys that form chromia scales. While much more expensive Ni-based or Co-based alloys and superalloys do exist that could be used for such applications, they cost 5-10 times more than commercial Fe—Cr—Ni austenitic stainless steels, and they would make such energy technologies cost-prohibitive.
Various alloying elements have effects on the complex microstructures produced in austenitic stainless steels during processing and/or during high temperature aging and service. The effects include changes in properties at high temperatures, including tensile strength, creep strength, rupture resistance, fatigue and thermal fatigue resistance, oxidation and corrosion resistance, oxide scale formation, stability and effects on sub-scale metal, and resistance to aging-induced brittleness near room-temperature.
A particular problem for use of stainless steels and alloys in such applications is that the fine grain size (<20-50 μm diameter) required to make thin section articles, completely changes the relative behavior of many alloys and/or the beneficial/detrimental effects of various alloying elements compared to heavier sections (ie. rolled plate or wrought tubing) with much coarser grain size. Fine grain size dramatically reduces creep resistance and rupture life, and below some critical grain size (1-5 μm diameter, depending on the specific alloy) the alloy is generally superplastic and not creep resistant at all. Two examples are 347 and 347HFG (high-carbon, fine-grained) and 347 and 310 austenitic stainless steels. As thicker plate or tubing, 347 HFG has twice the strength of 347, but as foils (nominal 3-10 mil thickness) with similar processing, 347 has better creep-rupture resistance than 347 HFG. Similarly, 310NbN stainless steel is much stronger than 347 steel as plate or tubing and has higher allowable stresses in the ASME construction codes, but as similarly processed foils, the 347 has significantly better creep-rupture resistance.
Therefore, fine-grained, thin-section manufacturing can dramatically reverse the relative strengths of various alloys and alter the expected microstructure properties thereof.